In the paper making industry, paper is ultimately formed from a mixture of fibers suspended in a solution of primarily water. Once the fibrous slurry is produced, it is fed to a headbox including a distribution system, a turbulence generating section, and a nozzle. Flow entering the headbox first enters the distribution system. The distribution system is typically utilized to normalize the properties of the slurry, such as the consistency, pressure, and velocity. Flow exiting the distribution system enters into the turbulence generating section, which is termed a diffuser section in hydraulic headboxes. The turbulence generating section delivers flow to a nozzle. The slurry then flows through the nozzle and is deposited in jet form onto a conveyer or wire. The slurry is then dried and treated to form the finished product.
It is important the slurry be sufficiently agitated while it is transported through the diffuser system and nozzle because the agitation keeps the fibers generally uniformly suspended throughout the fluid, and maintains random fiber orientation. If the fibers are not generally uniformly distributed (i.e., if areas of higher concentration exist), and are not randomly oriented (i.e., if some of the fibers are aligned) the resultant product may have "streaks," which are areas of structural variation in the paper. Streaks are manifested in paper sheets that are distorted such that the paper sheet does not lie flat.
FIG. 1 illustrates a typical prior art diffuser section wherein the slurry flow, or pulp flow is fed from the distribution system 64 to the diffuser section 50. The diffuser section 50 is comprised of a series of parallel, bundled tubes 52 that are anchored in an inlet wall 54. The incoming pulp flow 56 impinges upon the inlet wall 54 and ultimately enters the diffuser pipes 52 at an inlet 58. The flow through each of the diffuser tubes 52 is exhausted into a nozzle area (not shown). In order to maintain the agitation of the pulp, the walls of the tubes 52 typically diverge in the downstream direction (not shown) to create an adverse pressure gradient (i.e. a pressure gradient that decreases in the downstream direction). The pressure drop created at each tube inlet 58, in combination with the adverse pressure gradient, introduces small-scale turbulence which maintains fiber suspension in the slurry, and helps to randomize the fiber orientation and distribution. The diffuser section thereby agitates the fluid through "controlled turbulence".
As noted above, the tubes 52 are fit into holes formed in an inlet wall 54. The solid portions 60 of the wall are flow stagnation areas, and the tube inlets 58 are flow sinks. The distribution system 64 supplies the pulp flow 56 and is thus a flow source. A model for describing the arrangement of vortices in a two dimensional flow, known at the vonKarman vortex street, predicts that the symmetric positioning of stagnations (60) and sinks (58) in the presence of a source (56) introduces a series of oscillating vortices. Applicant has extended this concept to three dimensions, and applied the concept to headboxes. Applicant has discovered that vortices in the diffuser tubes may be created by dead spots 60 on the diffuser section wall 54. The vortices may then travel down each diffuser section tube 52 either in succession, or in a set pattern. When the vortices exit the diffuser tubes (indicated at 62), the exiting vortices create secondary flows, which are flows in a circular direction that causes the fibers to align and/or concentrate. The secondary flows align the fibers in the pulp, and repeated secondary flows cause the aligned fibers to appear as "streaks" in the finished paper products. As noted earlier, streaks are areas of structural variation in the paper believed to be caused by aligned of fibers. Streaks may cause mechanical condition defects in the papers, which can result in paper that will not lie flat (i.e., warped or buckled paper). Therefore, it appears that the presence of streaks may be attributed to the vortices 62, that in turn may be attributed to stagnation points 60 on the diffuser wall 54.
There have been prior art attempts to reduce or eliminate the effects of secondary flows. However, most of these efforts focus upon minimizing the effect of secondary flows, and have not addressed the source of secondary flows. Accordingly, there exists a need for a diffuser section which can minimize stagnation areas on the inlet wall and thereby minimize vortices that cause secondary flows and streaking in the finished paper.